The water-absorbent resin is widely used for sanitary materials such as paper diapers and sanitary goods, commodities such as pet sheets, industrial materials such as water-absorbent sheets for food, water stop materials for cables, and dew condensation preventing materials, water retention agents for greening, agriculture, and gardening, and a soil conditioner. The water-absorbent resin is particularly used for sanitary materials, among the uses described above.
Generally, such a water-absorbent resin is a slightly crosslinked polymer. As the water-absorbent resin, for example, a starch-based water-absorbent resin such as a hydrolysate of a starch-acrylonitrile graft copolymer (see Patent Literature 1), and a neutralized product of a starch-acrylic acid graft copolymer (see Patent Literature 2), a saponified product of a vinyl acetate-acrylic acid ester copolymer (see Patent Literature 3), and a partially neutralized product of polyacrylic acid (see Patent Literatures 4, 5, and 6) are known.
The water-absorbent resin can be produced by polymerizing a water soluble ethylenically unsaturated monomer in an aqueous liquid medium (liquid medium containing water), but since polymerization heat is generated during polymerization reaction, the polymerization heat has to be removed. As the method of removing the polymerization heat, a method of providing a jacket or a coil through which a cooling medium such as water flows, outside or inside of a polymerization reactor in which a polymerization reaction is performed in a water soluble ethylenically unsaturated monomer, a method of connecting a cooler such as a heat exchanging structure (reflux condenser) to the polymerization reactor (see Patent Literature 7), and a method for using evaporation latent heat of polymerization liquid are used.